In humans, the heart beats to sustain life. In normal operation, the heart pumps blood through the various parts of the body. Sometimes the heart malfunctions, in which case it can beat irregularly, or not at all. The cardiac rhythm is then generally called an arrhythmia. Some types of arrhythmia may result in inadequate blood flow, thus reducing the amount of blood pumped to the various parts of the body. Some arrhythmias may even result in a Sudden Cardiac Arrest (SCA). In a SCA, the heart fails to pump blood effectively and, if not treated, death can occur. In fact, the American Heart Association (AHA) reported in 2014 that SCA results in more than 500,000 deaths per year in the United States alone. Further, a SCA may result from a condition other than an arrhythmia. One type of arrhythmia associated with SCA is known as Ventricular Fibrillation (VF). VF is a type of heart malfunction where the ventricles make rapid, uncoordinated movements, instead of the normal contractions. When that happens, the heart does not pump enough blood to deliver enough oxygen to the vital organs. The person's condition will deteriorate rapidly and, if not reversed in time, they will die soon, e.g. within ten minutes.
Ventricular Fibrillation can often be reversed using a life-saving device called a defibrillator. A defibrillator, if applied properly, can administer an electrical shock to the heart. The shock may terminate the VF, thus giving the heart the opportunity to resume pumping blood. If VF is not terminated, the shock may be repeated, often at escalating energies.
A challenge with defibrillation is that the electrical shock must be administered very soon after the onset of VF. There is not much time: the survival rate of persons suffering from VF decreases by about 10% for each minute the administration of a defibrillation shock is delayed. After about 10 minutes, the rate of survival for SCA victims averages less than 2%.
During VF, the person's condition deteriorates, because blood is not flowing to the brain, heart, lungs, and other organs. Blood flow must be restored, if resuscitation attempts are to be successful.
Cardiopulmonary Resuscitation (CPR) is one method of forcing blood flow in a person experiencing cardiac arrest. In addition, CPR is the primary recommended treatment for some patients with some kinds of non-VF cardiac arrest, such as asystole and Pulseless Electrical Activity (PEA). CPR is a combination of techniques that include chest compressions to force blood circulation, and rescue breathing to force respiration.
Properly administered CPR provides oxygenated blood to critical organs of a person in cardiac arrest, thereby minimizing the deterioration that would otherwise occur. As such, CPR can be beneficial for persons experiencing VF, because it slows the deterioration that would otherwise occur while a defibrillator is being retrieved. Indeed, for patients with an extended down-time, survival rates are higher if CPR is administered prior to defibrillation.
CPR chest compressions must be administered continuously, or nearly so. Methods have been devised for assessing a rescue team's performance, so as to help them improve their training. A method by Physio-Control, Inc. is described in U.S. Pat. No. 8,080,199 B2, which is hereby incorporated by reference. One or more figures of merit are computed about the administered CPR. Figures of merit can be metrics.
A specific metric that has received much focus and emphasis is the chest compression fraction, which is also known as “hands-on time”, chest compression ratio, CPR ratio, flow time, etc. The chest compression fraction represents a ratio of the total amount of time that chest compressions were performed during an interval of interest, up to and including an entire resuscitation, over the total duration of that interval of interest.
The chest compression fraction is attractive because it distills what is thought to be a dominant aspect of CPR quality down to a simple interpretable number. That aspect is the answer to “for how much time were chest compressions actually being done while the patient needed them?” While that is true, a problem remains that interruptions in the chest compressions are not properly accounted for.